Did you took some time to browse the contents of this site?
if so you should have seen that it has very little in common with crime and everyone has very different tastes and preferences and your questions are only relevant to very specific kind of people here. How about those who like consensual guro? or amputees? Or cannibalism?

But if you are interested in my opinion
I am not member of any true crime communities but watch real stuff sometimes. although just because of curiosity

This idea that guro is a fantasy is typical cop-out for people who have internal moral conflict. People who claim it to be fantasy are usually ashamed of their interests and fiercely blindly attack their own interest in the real life. Same as those closeted gays being the most extreme gay haters. I am in general against any punishments. Punishment is nothing else than plain torture/revenge.

I don't think guro is attractive in the same as public beheadings in medieval ages, at that time was the only kind of entertainment that people had what else will your watch when your life is so boring that you woud probably woud not mind be be burned on that stake yourself :)
I think for me it is attractive because I like everything weird.

Also my own theory is that we almost all like self destructive behavior and find it sexually attractive. To the extent where sexuality is by itself considered destructive behavior. Guro is probably on the most extreme side of this.
Some people also seem to prefer non-con destruction like consensual sex vs rape or other form of coercion.